The present invention relates to an illumination system, for a microlithographic exposure apparatus having a diffractive optical element to reshape a radiation beam, and to said diffractive optical element. More particularly, the invention relates to the application of such a device in a lithographic projection apparatus.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation and such elements may also be referred to below, collectively or singularly, as a “lens”. Any refractive, reflective or catadioptric elements in the radiation or illumination systems may be based on a substrate of glass or other suitable material, and may be provided with either single- or multi-layer coatings as desired. In addition, the first and second object tables may be referred to as the “mask table” and the “substrate table”, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatus are described in International Patent Applications WO98/28665 and WO98/40791.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a waferstepper. In an alternative apparatus—which is commonly referred to as a step-and-scan apparatus—each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally ≦1), the speed ν at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
In one form of microlithography, a mask defining features is illuminated with radiation from an effective source having an intensity distribution at a pupil plane corresponding to a particular illumination mode. An image of the illuminated mask is projected onto a resist-coated semiconductor wafer.
One method to reduce feature size, i.e. increase resolution, in optical lithography, is off-axis illumination. With this technique, the mask is illuminated at non-perpendicular angles which may improve resolution, but particularly improves the process latitude by increasing the depth of focus and/or contrast One known illumination mode is annular, in which the conventional zero order spot on the optical axis is changed to a ring-shaped intensity distribution. Another mode is multipole illumination in which several spots or beams are produced which are not on the optical axis. The spatial intensity distribution at the pupil plane is converted into an angular distribution at the mask plane.
Problems with the prior art include lack of flexibility of the illumination system such as only having fixed illumination modes or a limited range of modes or difficulty in selecting or mixing desired modes. Some prior systems also have a high loss of energy by blocking parts of the illuminating radiation.